This invention relates to a focal-plane shutter for a digital camera wherein to photograph, two blades consisting of a first blade and a second blade are operated in order in the same direction and the light reception face of an image pickup device is exposed to light through a slit formed by the first and second blades.
Generally, with a focal-plane shutter adopted for a conventional camera using a silver-salt film, first blades electromagnet and second blades electromagnet are previously energized before the light exposure operation of a first blade and a second blade. Then, the electromagnets is shut off in order at a predetermined timing by an output signal of a light exposure time control circuit, and a first blades drive member and a second blades drive member are rotated by the urging forces of a first blade drive spring and a second blade drive spring, thereby causing the first blade and the second blade to perform the light exposure operation.
Two types of configuration for holding the first blades drive member and the second blades drive member at the set position to the next photographing are known, usually called direct type and retention type. In the direct type, at the set position, the first blades drive member and the second blades drive member are brought into contact with the first blade electromagnet and the second blade electromagnet respectively, and when a release button of the camera is pressed at the next photographing, the electromagnets are energized and at the stage at which the drive members are reliably attracted by the magnetic forces of the electromagnets, a set member is restored to the initial position before the set operation.
In the retention type, the first blades drive member and the second blades drive member are retained at the set position by retention members and when a release button of the camera is pressed at the next photographing, the retention is released by the electromagnets. Therefore, in the retention type, a set member may be restored to the initial position immediately after completion of setting the drive members, or may be restored to the initial position before photographing in association with release as with the direct type described above.
By the way, a digital camera for photographing a subject by an image pickup device of a CCD, etc., for converting photograph light formed by a photographing lens into electric data adopts a focal-plane shutter of a similar type to the type described above, in which a first blade and a second blade are operated in order at a predetermined timing and continuous light exposure is conducted from one side of a light reception face of the image pickup device to an opposite side through a slit formed by the first and second blades.
Some of the digital cameras using a shutter of such a configuration are of the type in which an optical finder is provided as with the conventional camera and the light reception face is covered with at least either of the first and second blades except that photographing is conducted, and of the type in which a subject can be displayed by an electronic viewfinder implemented as a liquid crystal display, etc., for making it possible to view image data output from the image pickup device with the image pickup device exposed to photograph light even when photographing is not conducted. Therefore, in the latter type, to photograph, the first blade is operated to the position covering the light reception face before light exposure operation is performed.
FIG. 5 shows a related art as disclosed in JP-A-62-128675. The related art discloses a shutter of the direct type described above. When the camera power is turned on, as a motor lever 107 (operation member) is turned clockwise, a set lever 109 (set member) is turned counterclockwise from the initial position. Thus, an open blade drive lever 101 (first blades drive member) and a closed blade drive lever 104 (second blades drive member) are turned clockwise against the urging forces of a spring 103 (first blade drive spring) and a spring 106 (second blade drive spring) and the set completion state is entered. At this time, only the closed blade drive lever 104 is attracted to an electromagnet on the closed vane side (second blade electromagnet). After this, as the set lever 109 is restored to the initial position, the open blade drive lever 101 is operated to the open position, but the closed blade drive lever 104 is already attracted to the electromagnet and thus an aperture (opening) is opened by an open vane (first blades) and a closed vane (second blades) and it is made possible to display a subject through an electronic viewfinder.
When release operation is performed, the electromagnet on the closed vane side is demagnetized and thus the closed blade drive lever 104 is turned counterclockwise by the tension of the spring 6 for moving the closed vane to the position closing the aperture. After this, in a state in which both an electromagnet on the open vane side (first blade electromagnet) and the electromagnet on the closed vane side are excited as the set lever 109 is turned counterclockwise, the open blade drive lever 101 and the closed blade drive lever 104 are operated to the set completion position and are attracted to their respective electromagnets. After the open blade drive lever 101 and the closed blade drive lever 104 are thus attracted and held, when the set lever 109 is restored to the initial position by the urging force of a spring 111, the electromagnet on the open vane side and the electromagnet on the closed vane side are demagnetized in order, whereby the blades start the light exposure operation.
In such a control method, when the camera is not used, the shutter blade is closed to protect the image pickup device from direct sunlight and as the power is turned on, the aperture is opened; this point is desirable. On the other hand, however, from the time when the power is turned on, the closed blade drive lever 104 must be kept at the open position to display a subject on the electronic viewfinder and thus an electric current must be continuously supplied to the closed vane electromagnet and power consumption is noticeable; this is an extreme problem.
If the open position is kept by the attraction force of the closed vane electromagnet, when vibration or a shock is applied to the camera, it is transmitted to the shutter, whereby the closed blade drive lever 104 is not kept at the open position and is brought away from the open position and a malfunction of closing the aperture may occur. Thus, a countermeasure of increasing the electric current applied to the electromagnet for enlarging the attraction force is taken; however, also in this case, a power consumption problem occurs. Particularly, with the digital camera, the camera size is made small as compared with that of the conventional camera and thus the power supply volume is limited and photoelectric conversion of the image pickup device, display on the electronic viewfinder, image recording, and the like also consume power and thus there is strong demand to suppress power consumption as much as possible. Further, to suppress costs, the focal-plane shutter of the conventional camera may be used as that of the digital camera. In such a case, however, it is large demand to suppress change in the shutter configuration and change in the parts accompanying the change in the shutter configuration as much as possible for reducing costs together with suppression of power consumption described above.
It is an object of the invention to provide a focal-plane shutter for a digital camera for exposing the light reception face of an image pickup device to light through a slit formed by first and second blades, the focal-plane shutter being suited for a camera comprising an electronic view finder and appropriate for lower power consumption and cost reduction.
To the end, according to the invention, there is provided a focal-plane shutter for a digital camera, comprising a shutter base plate having an aperture for light exposure; a set member being capable of reciprocating on the shutter base plate, when the set member is moved by an operation member on the camera side and is operated from an initial position to a set position, the set member for operating a first blades drive member for driving a first blade and a second blades drive member for driving a second blade to the set position against urging forces of drive springs and before light exposure operation of each drive member starts, the set member being restored to the initial position in association with the operation member; and a first blade holding member and a second blade holding member for holding each drive member at a light exposure operation start position just before the light exposure operation starts and when the light exposure operation starts, releasing holding each drive member at a predetermined timing, wherein
when the set member is operated from the initial position to the set position, at the initial stage, the set member operates at least the second blades drive member against the urging force of the drive spring and once stops at a midway position at which the second blade opens the aperture in association with motion of the operation member.
In the focal-plane shutter of the invention, if the slit formation margin of the first blade is made to enter the aperture to a position not covering the light reception face of an image pickup device placed on a photograph optical path passing through the aperture by the time the set member reaches the midway position from the initial position, the operation amount of the first blade from release to the set position is lessened, so that it is made possible to shorten the time required for the set operation. If the first blade holding member and the second blade holding member are each an electromagnet for attracting an iron piece member placed on each drive member, the configuration of the shutter is simplified.